Modern semiconductor processes enable manufacturing of semiconductor structures having complex geometric features. Electrical and optical properties across the geometrical features, may be engineered in accordance with requirements of particular applications.
By way of an example, a P-N junction that exhibits a U-shaped cross-sectional profile may be manufactured across an optical waveguide. The U-shaped structure has one dopant as the U, and another dopant encroaching at a center of the U. A U-shaped P-N junction may be used, for instance, to improve the performance of in a silicon-based Mach Zehnder modulator. The U-shaped P-N junction may extend across the surface of the silicon substrate to form a complex structure.
Prior techniques to create such a structure rely on introducing dopants in a series of application steps, at an acute angle of dopant delivery relative to the substrate. In the example of a U-shaped junction, the substrate is tilted relative to the dopant delivery source in steps, to allow for a plurality of doping delivery steps at different dopant angle trajectories. This allows the dopant to be deposited about the curve of the bottom of the U-shape. Similar techniques may be used for other three-dimensional structures.
Altering the angle of dopant delivery complicates the manufacturing process, leading to increased time and cost of manufacture. Depending upon the structure being built, using multiple delivery steps at different delivery angles may be difficult or impossible to use, depending upon the layout of the structure. For instance, using a non-perpendicular delivery angle on a curved or angled structure will result in different doping amounts being received along the structure.
Acute angle doping may give unsatisfactory results when the feature being doped is curved on the substrate surface. For example, ring modulators are formed having a junction in the cross-section of the ring that extends around the annulus of the ring.
It would be very difficult, if not impossible, to manufacture a ring modulator using a three-dimensional junction (such as a U-shaped junction, for instance) if the dopant delivery angle needs to be altered during fabrication. The difficulty is that a ring modulator has waveguide bends and/or is rotationally symmetric about a central axis. Changing the delivery angle for one radial segment of the ring would result in a different delivery trajectory for the rest of the ring.
Other features may deviate from a straight line across the substrate surface, which present similar difficulties when altering the angle of delivery. For instance, Mach-Zehnder modulators may follow an angled or arcuate path as the interferometer arms are combined. These structures may similarly pose difficulties when attempting to form two junctions in close proximity. Tilting the substrate to change the angle of dopant delivery makes it difficult to separately apply or mask the dopant delivery between the two junctions.
There is a need for a system and method for manufacturing semiconductor structures that overcome some of the limitations of the prior art.